Orthogonal frequency division multiplexing system is a multi-carrier transmission technique that uses orthogonal subcarriers to transmit information within an available spectrum. Since the subcarriers are orthogonal to one another, the subcarriers can be spaced much more closely together within an available spectrum than, for example, the individual channels in a conventional frequency division multiplexing (“FDM”) system. Many modern digital communications systems are turning to the OFDM system as a modulation scheme for signals that need to survive in environments having multipath-propagation or strong interference, including the IEEE 802.11a standard, the Digital Video Broadcasting Terrestrial (“DVB-T”) standard, the Digital Video Broadcasting Handheld (“DVB-H”) standard, the Digital Audio Broadcast (“DAB”) standard, and the Digital Television Broadcast (“T-DMB”) standard.
In an OFDM system, the subcarriers may be modulated with a low-rate data stream before transmission. It is advantageous to transmit a number of low-rate data streams in parallel instead of a single high-rate stream since low symbol-rate schemes suffer less inter-symbol interference (“ISI”) caused by multipath.
OFDM modulated signals can be transmitted in transmission frames, where each transmission frame consists of a number of symbols. The reception of these signals depends on successful acquisition of symbol timing and frame timing. Symbol timing acquisition can be accomplished by finding the boundary of each symbol; whereas frame timing acquisition can be accomplished by finding the starting symbol of each transmission frame.
In particular, with respect to OFDM modulated signals, timing synchronization and frequency synchronization are difficult. It is difficult to exactly synchronize symbols between the transmitter and the receiver. Timing synchronization requires that the beginning of each OFDM symbol be determined within each frame. Unless the correct timing is known, the receiver cannot remove cyclic prefixes at the correct timing instance. Thus, individual symbols cannot be correctly separated before a Fast Fourier Transform (“FFT”) is applied to demodulate the signal.
In a wireless environment with multipath reception, finding the optimal FFT window timing can result in the lowest inter-symbol-interference (“ISI”), and therefore the best receiver performance. Fine timing synchronization serves this purpose. To find the timing window, i.e., the start of an FFT window, a conventional method is to locate the strongest path via a time domain correlation or inverse FFT and then place the FFT window a few samples shift from the strongest path. In an additive white Gaussian noise (“AWGN”) environment, this scheme works just fine since the delay is very limited. However, in a dynamic environment with various multipath delays and multipath profiles, such methods are not adequate. Therefore, it is desirable to provide methods for timing synchronization for OFDM modulated signals.